BUYING health products from Amazon and other shopping websites because of good reviews can be a mistake, according to Scottish research.

Psychologist Dr Micheal de Barra said the posts do not provide an accurate reflection of their actual benefit because they portray them in a far more positive light than clinical trial data would suggest.

The research fellow at Aberdeen University said this was because people are much likelier to share a positive outcome rather than a negative one.

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In the first study of its kind, he examined more than 1,600 online reviews of weight loss pills and high cholesterol treatments sold on on Amazon.com.

He found the average drop in cholesterol using buttery spread Benecol – which says it is proven to lower cholesterol – was more than three times greater than in carefully controlled studies.

Similarly, reviewers on weight loss pill Orlistat – sold over-the-counter as Alli in the UK – lost about twice as much weight (31lbs) as those in clinical trials (15.5lbs).

Dr de Barra said: “These treatments are not entirely ineffective.

“However, what we show is the reputation as described in these reviews is much more positive than the clinical trial data show.”

He explained reviews are not a deliberate attempt to mislead, but appear exaggerated as a result of a bias towards sharing good news.

Dr de Barra, who has an interest in historical and contemporary inaccurate medical beliefs, said: “Only some people who try a treatment will then go on to tell other people about their experience.

“However, this subset of people are usually only those who have good outcomes. So, you hear a friend of yours had a good result using a treatment of some kind, and you think ‘well maybe this works’.

“Your friend is probably not lying, but the problem is people with average or poor outcomes do not tend to share their experiences. This means you get a positively skewed view of the treatment.”

He said the study, published in the journal Social Science and Medicine, suggest people should be careful when it comes to review sites. It is believed to be the first study to compare clinical trial data with user generated content.

Dr de Barra warned: “We should be cautious about using reviews like these when deciding about health choices.

“These narratives have a powerful influence on our own future health behaviour because they provide simple and clear anecdotes, but this study shows they can be very misleading.

“These results also shine a new light on medical overuse, the use of treatments that are unnecessary and ineffective.

“Medical overuse is estimated to cost the $226bn (£180bn) in the US alone, and patient demand for medicines with limited value is one important cause.

“This study shows how a demand for ineffective medicines can easily develop when people rely on hearsay and narratives alone.

“Realistically, however, it is ridiculous to think every health decision we make will be informed by systematic reviews – people have lives to lead.

“This study shows it is important to be aware of the biases that can make informally acquired information unreliable.”